In many separation processes, especially in the case of drugs and pharmaceuticals, it is important to obtain a product having a high degree of purity. To this end it is also important, where countercurrent extraction is used, to provide high selectivity, stages having maximum equilibrium efficiency and maximum quantitative separation and recovery of the desired product at minimum time and expense.
It is known from the teachings of U.S. Pat. Nos. 4,336,106 to Winter, III (1982); 4,732,685 to Brandt et al. (1988); 3,898,291 to Darsi (1975); and 3,427,357 to DeGramont et al. (1969) to use liquid-liquid extraction columns having a number of successive stages of mixing and settling zones, e.g., of alternating mechanical agitating or flow diverting mixing areas and packing material or other constructional element settling areas, or perforated tray arrangements, or the like, to separate a given solute by preferential extraction in one of a heterogeneous mixture of immiscible solvents passing as two separate phases in countercurrent flow through the column.
The recent teaching of U.S. Pat. No. 4,632,809 to Otto et al. (1986) exemplifies an analogous system used for differential temperature crystallization/separation of a given solute from a liquid in an open flow vertical column, cooled along its upper half and heated along its lower half, and equipped with a rotating scraper to dislodge into the heated liquid those crystals forming on the cooled upper half of the column wall.
Per the former teaching of U.S. Pat. No. 2,631,780 to Lewis (1944), a countercurrent flow series of horizontal spray mixing and phase settling contiguous zones is operated to extract a given solute with a mixture of miscible liquids and, per the former teaching of U.S. Pat. No. 2,250,976 to Van Dijck (1941), a countercurrent flow column is also operated to extract a given solute with a mixture of miscible liquids, but in both cases, like the above noted recent teachings, these systems are operated while pointedly maintaining the liquids as a 2-phase heterogeneous mixture of a continuous phase and a dispersed (controlled size droplet) phase, such that a 1-phase homogeneous (totally miscible) mixture of the liquids is never formed.
The difficulty with these known liquid-liquid extraction systems, particularly where it is desirable to extract a high molecular weight compound (e.g., a drug) or separate a high molecular weight impurity, is the inefficient and consequently slow mass transfer of the solute or solutes between the solvent streams. The problem is particularly aggravated in biotechnology where the liquid-liquid extraction processes are a significant part of production cost and therefore of major economic significance.